Propulsion systems utilizing cryogenic liquid oxygen and/or hydrogen, such as the Space Shuffle, Atlas/Centaur, Delta, etc., are currently filled from the facility storage tanks and subsequently allowed to cool in the flight tanks in order to reject the heat absorbed by the liquid as a result of environmental heat leak, transfer line, and tank wall chill-down. The cooling of the liquid bulk is desirable in order to increase the liquid density so that more impulse mass can be stored in the tank, and also to reduce the liquid vapor pressure so that the tank operating pressure and tank weight is minimized.
Heat rejection from the liquid bulk is a relatively slow process since it depends on natural convection mechanism and liquid surface evaporation. The degree of liquid cooling through surface evaporation is also limited by the vent system flow resistance (vent valve and line) and the ambient pressure (14.7 psia). Reducing the vent system flow resistance to improve the cool down time and minimize the final bulk temperature results in a relatively large vent valve and line design which represents a vehicle payload weight penalty. Although the current means of densifying the cryogenic liquids through evaporation are simple the process is limited to the saturation density and liquid vapor pressure at one atmosphere.